Liquid mixtures with suspended particles, such as slurries, facilitate many industrial processes. One common industrial process in which slurries are utilized is the precision lapping and polishing of thin semiconductor wafers by semiconductor processing machines, such as a lapping machine. Slurries used in lapping and polishing semiconductor wafers typically are thin mixtures of an abrasive, such as aluminum oxide, and water.
In the lapping process, a lapping machine typically supports the wafer in a carrier, sandwiched between two parallel lapping plates. The lapping machine distributes the abrasive slurry to the semiconductor wafer at various locations along the lapping plates, such that the slurry forms a thin, even layer between the lapping plates and the semiconductor wafer. Rotating action of the carrier between the lapping plates causes the semiconductor wafer to grind against the abrasive slurry, thereby smoothing the surface of the semiconductor wafer.
Slurry may be supplied to a lapping machine by a variety of slurry supply systems. For small-scale operations involving one lapping machine or a group of lapping machines located in a small area, a portable container of slurry may be placed near the lapping machines such that a pump on each lapping machine may draw slurry out of the portable container and into each lapping machine. Because particles tend to settle out of a slurry when the slurry stagnates, a mechanical agitator is typically used in such a portable container to keep the particles suspended in the slurry. However, mechanical agitators are expensive and contain moving parts that require service after repeated use.
For large-scale production of silicon wafers, it is common for several lapping machines to receive a supply of slurry from a common reservoir. The lapping machines are often distributed throughout a factory complex, and may be located remote from the reservoir, such as on a separate level of the factory vertically distant from the reservoir. To supply distributed lapping machines with slurry in such a large-scale setting requires a supply conduit extending from the reservoir to each machine. A supply conduit pump pumps the slurry from the reservoir to each lapping machine through the supply conduit. The supply conduit typically runs through a factory floor, and supplies slurry to lapping machines located on the factory floor above the supply conduit.
In current large-scale slurry supply systems, the supply conduit pump pressurizes the supply conduit. Delivery lines in the form of flexible tubing attach to headers located on the pressurized supply conduit at locations proximate each lapping machine. The headers on the supply conduit allow some pressure from the supply line to be transferred through the header such that slurry flows out the header and into the flexible tubing. Peristaltic pumps on the top of each lapping machine act in conjunction with the pressure from the supply conduit to draw slurry up and into each lapping machine.
Several problems exist with such pressurized supply conduits. First, where the reservoir is vertically distant from the lapping machines, great pumping force is required to pressurize the supply conduit. Under such strain, the supply conduit pump wears out at an accelerated pace. Repeated replacement of the supply conduit pump is costly, as is outfitting the supply conduit with a larger pump capable of generating adequate pressure.
In addition, inadequate pressure in the supply conduit, such as occurs when the supply conduit pump is wearing out or straining to pressurize the supply conduit, lowers the efficiency of the peristaltic pumps drawing slurry from the supply conduit to each of the lapping machines. The peristaltic pumps experience low efficiency because the supply conduit pump conveys slurry at a slow rate into the flexible tubing, causing the peristaltic pumps to pump less slurry with each stroke on the flexible tubing than would be pumped if slurry was conveyed at a faster rate. To achieve a desired flow into the lapping machine, the low-efficiency peristaltic pumps must be run at higher stroke rates, resulting in a shorter pump life for the peristaltic pumps. Higher stroke rates, in turn, cause the flexible tubing to wear and crack prematurely.